The present invention relates to vessels and vascular support structures, such as stays, stents and support rings which are used for maintaining open a biological passage, such as an artery. A great many constructions for both prostheses and stents are known in the art, ranging from simple wire or plastic rings and arrangements of stiff but flexible sheets or shells, to technologically advanced constructions wherein a wire structure of heat memory alloy flips to an enlarged memory configuration, or wherein a solid tubular body is fabricated with microscopically thin laser-cut slots which convert the solid cylinder into an expandable body that opens out to form a mesh-like but reasonably stiff surface support. One commercial embodiment of this latter type of stent, referred to as the Palmaz stent after the surgeon who popularized this construction, is in common use now. Another common form of stent consists of wire crimped into a zig zag pattern which can be expanded to attain a much larger length or diameter. Stents of this form may be formed as individual rings, or serpentine windings, or as pairs of helical windings which act against each other to counterbalance twist while expanding radially. Numerous other constructions are known.
Many if not all of the materials used for stents involve metal or carbon fiber materials which are highly electro-positive and are bio-active. Since stents tend to be used under conditions where they are counteracting disease processes, supporting healing processes, or guarding against stenosis of a passage, bio-activity, which may encourage undesirable or poorly regulated growth processes, or lead to clot formation, should be avoided. Coating of stent can keep the stent from directly contacting surrounding tissue or fluids, and thus can theoretically protect against unwanted electrochemically induced tissue reactions.
In the field of expandable stents, however, a further problem arises due to the fact that many effective or compact stent constructions involve filamentous or wire-like structures which have numerous apertures or spaces between the various stands or structural elements of the stent. With these constructions, tissue may grow through the openings of the stent Furthermore, the stent itself may provoke a foreign body reaction and be both a stimulus for and a framework supporting, proliferative tissue growth, resulting, for example, in scar tissue or restenosis of the very region it is placed to control.
One approach to this drawback is to provide a coating, liner or cover for the stent which prevents the healing or diseased layer of tissue from directly contacting the stent or from passing through the stent in any way. Such liners may be formed, for example, of porous polytetrrafuoroethylene (PTFE) which allows the passage of fluids and vital materials while serving as a barrier to tissue growth. However, when applying such a construction, a further difficulty which may arise is that the layer or sleeve of polymer must be attached to the stent for example, by staples or sutures at one end, or is prone to developing loose pockets or folds which might accumulate organic matter or lead to sepsis or unusual growth. Also, the necessarily thin liner material may detach or degrade. The risk of loose or unattached liner material is particularly great for constructions which utilize poorly adherent polymers, such as PTFE, or structures which seek to combine an expandable stent of stiff material, which changes both its dimension and its shape, with a dissimilar liner or shell.
Accordingly, there remains a need for a covered support construction of enhanced hardiness and implant compatibility.
There is also a need for an expandable vessel support which forms a unitized and non-delaminating tissue barrier.
There is also a need for a need for a vascular liner having atraumatic properties and haemodynamic shape.